Gruesome Tale: Why Wasps Live Inside Zombie Ladybugs

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If a ladybug's life were a horror film, this is how it would
start: Scary string music. A close-up of the green-eyed face of a
wasp. The sudden pierce of a stinger. The screen goes dark.

Next, an establishing shot of our ladybug hero, sitting placidly
on a leaf. Suddenly, the sky clouds over. Something orange
and grubby begins to poke from the ladybug's abdomen.
Audience members cover their eyes, expecting a quick, gruesome
end for the black-and-red insect. But it's not that easy. Instead
of dying, the ladybug survives as a wasp larva emerges from its
abdomen and begins to weave a cocoon between the ladybug's legs.
That's right: The ladybug is a zombie.

This sordid tale isn't fiction for many ladybugs that fall victim
to the parasitical wasp Dinocampus coccinellae. Now, a
new study reveals why the wasps use ladybugs as incubators. It
turns out that the zombie ladybugs keep predators away from the
wasps' vulnerable larva, increasing the likelihood that they
survive to become full-fledged wasps.

The research, published today (June 21) in the journal Biology
Letters, finds that this protection comes at a cost: Larva that
cocoon themselves to a living ladybug, as opposed to a dead one
or to none at all, can expect fewer eggs of their own when they
emerge as wasps. This suggests that the same resources the wasps
use to develop their eggs are also used to control the zombie
ladybug. [Read:
The 10 Most Disgusting, Diabolical Parasites ]

Ladybug horror

The wasps' parasitical ways have been long noticed, and they
aren't unique in the insect world. The parasitic wasp
Hymenoepimecis argyraphaga, for example, lays its eggs
in the spider Plesiometa argyra. The larvae then eat
their way out of their host.

Nor is mind control very extraordinary for parasites. Before it
dies, an infected P. argyra spider is compelled to build
its web in a good location for a wasp cocoon.
Zombie ants infected by a certain fungus wander around the
forest until high noon, when they anchor themselves to a leaf
vein with their jaws. At sunset, the ants die as the fungus sends
a stalk shooting through the tops of their heads. [ Mind
Control: Gallery of Zombie Ants ]

But University of Montreal graduate student Fanny Maure and her
colleagues noticed that even after D. coccinellae larvae
burrow their way out of ladybugs' bellies, the ladybugs stay
alive, partially paralyzed but twitching occasionally. They
suspected that the living ladybugs might be somehow protecting
their uninvited cargo. [ See
photos of the ladybugs and their parasites ]

To test the idea, the researchers reared more than 4,000 ladybugs
in the lab and let wasps lay their eggs in the unfortunate
insects. They then waited for the larvae to emerge and spin their
cocoons.

Ladybug bodyguard

Once the larvae were out, the researchers split the zombie
ladybugs and the larvae into three groups. In the first, they
removed the ladybug from the cocoon, leaving the developing
cocoon alone. In the second, they left the cocoon on the ladybug,
but crushed the ladybug's head to kill it. The third group was
left as it was, with a developing wasp attached to a living
ladybug.

Then, they exposed all three groups of cocoons to green
lacewings, an insect predator that loves to chow down on
vulnerable wasp larvae. Each predator was allowed 15 minutes
alone with a cocoon as the researchers recorded how often the
lacewings were successful in snagging a larval meal.

They found that of the cocoons protected by
living ladybugs, only about 35 percent got eaten. When
cocoons were left alone or attached to dead ladybugs, in
contrast, between 85 and 100 percent fell prey to the lacewings.

The study also found that the longer the ladybug survived with
the cocoon attached to it, the less fertile the newly
emerged wasp was likely to be, suggesting the developing wasp
shares its resources with its host. Makes sense, as the more
resources the ladybug has before a larva hatches out of its
abdomen, the longer it lives to protect the larva in the cocoon.

More research is needed to find out whether the wasps develop
more eggs later in life to compensate for sharing their resources
with their zombie hosts, Maure wrote. But the study also turned
up another horrifying curiosity about zombie ladybugs: About 25
percent of the ladybugs survived the parasite process and went
about their lives once the wasp larva was gone. Now there's a
horror movie with the potential for a happy ending.